1. Field of the Invention
The present invention relates to injectors and atomizers, and more particularly to staged pilot fuel injectors for gas turbine engines.
2. Description of Related Art
A variety of devices and methods are known in the art for injecting fuel into gas turbine engines. Of such devices, many are directed to injecting fuel into combustors of gas turbine engines while reducing undesirable emissions. With the increased regulation of emissions from gas turbine engines have come a number of concepts for reducing pollutant emissions while improving the efficiency and operability of the engines.
Modern gas turbine engine designs include providing high temperature combustion temperatures for thermal efficiency throughout a range of engine operating conditions. High temperature combustion minimizes emissions of some undesired gaseous combustion products, such as carbon monoxide (CO) and unburned hydrocarbons (UHC), and particulates, among other things. However, high temperature combustion also tends to increase the production of nitrogen oxides (NOX). Thus measures must be taken to provide thermally efficient operation within a temperature range that minimizes NOX, CO, and UHC.
One method often used to reduce unwanted emissions is staged fuel injection, wherein the combustion process is divided into two (or more) zones or stages, which are generally separated from each other by a physical distance, but still allowed some measure of interaction. Each stage is designed to provide a certain range of operability, while maintaining control over the levels of pollutant formation. For low power operation, only the pilot stage is active. For higher power conditions, both the pilot and the main stages may be active. In this way, proper fuel-to-air ratios can be controlled for efficient combustion, reduced emissions, and good stability. The staging can be accomplished by axial or radial separation. Staged fuel injectors for gas turbine engines are well known in the art.
It is difficult to provide thermally efficient, low emissions operation over the widening range of conditions in gas turbine engine designs. Additionally, during low power operating conditions, conventional staged fuel injectors only have fuel flowing through one of the staged fuel circuits. Measures must be taken to control temperatures in the stagnant fuel circuit to prevent coking within the injector. In the past, attempts were made to extend injector life by passively insulating, actively cooling, or otherwise protecting the fuel circuitry of fuel injectors from carbon formation during low power engine operation.
Along with staged combustion, pollutant emissions can be reduced by providing a more thoroughly mixed fuel-air mixture prior to combustion wherein the fuel-to-air ratio is below the stoichiometric level so that the combustion occurs at lean conditions. Lean burning results in lower flame temperatures than would occur with stoichiometric burning. Since the production of NOX is a strong function of temperature, a reduced flame temperature results in lower levels of NOX. The technology of directly injecting liquid fuel into the combustion chamber and enabling rapid mixing with air at lean fuel-to-air ratios is called lean direct injection (LDI).
U.S. Pat. No. 6,389,815 to Hura et al. describes a lean direct injection system, which utilizes radially-staged combustion within a single injector. The pilot fuel delivery is of the “swirl-cup” type shown in U.S. Pat. No. 3,899,884 to Ekstedt, wherein a pressure swirl atomizer sprays liquid fuel onto a filming surface where the liquid film is stripped off into droplets by the action of compressor discharge air. The main fuel delivery system utilizes a series of discrete atomizers that spray radially outward into a swirling cross-flow of air. The main fuel delivery is staged radially outboard of the pilot, and operates in the fuel-lean mode. Separation of the pilot combustion zone from the main combustion zone is achieved by radial separation as well as an air jet located radially between the two combustion zones.
U.S. Pat. No. 6,272,840 to Crocker et al. discloses a lean direct injection system, which also utilizes radially-staged combustion within a single injector. The pilot fuel delivery is of either a simplex air-blast type, or a prefilming air-blast type, and the main fuel delivery system is of a prefilming air-blast type. The radial staging of the pilot and main combustion zones is achieved by ensuring that the pilot combustion zone remains on-axis with no central recirculation zone.
U.K. Patent Application No. GB 2 451 517 to Shui-Chi et al. describes a pilot circuit divided into a primary and secondary fuel split. The primary circuit includes a pressure atomizer (simplex) on the centerline that is used for low power operation. The secondary pilot circuit is radially outboard of the primary circuit and is in the form of circumferentially spaced ports aimed towards the centerline. These circumferentially spaced ports are prone to external and internal carbon concerns.
Pure airblast nozzles are in wide use among engine manufacturers, particularly in aircraft engines. Pure airblast nozzles create favorable air/fuel mixtures and have spray characteristics that produce combustion qualities desirable for low emissions and high engine efficiencies. A typical pure airblast injector has one fuel circuit. Fuel can be directed from an injector inlet fitting to a fuel swirler through a fuel tube. At the fuel swirler, the fuel can be split into a multitude of discrete paths, all of which discharge into the combustor. These discrete paths are all fluidly connected and are thus all part of a single fuel circuit.
One example of a pure airblast fuel injector is described in U.S. Pat. No. 6,622,488 to Mansour, et al., which shows a fuel injector having a pure airblast nozzle connected to a housing stem. The fuel nozzle includes a fuel swirler that has a plenum for receiving fuel from a conduit in the housing stem. A plurality of fuel passages conduct fuel from the plenum to discharge orifices. The downstream ends of the passages are angled so as to impart swirl on fuel exiting therethrough. A prefilmer surrounds the fuel swirler. Fuel exiting the swirler is directed inwardly by the prefilmer. An inner air passage extends through the center of the fuel swirler and an outer air passage is defined outboard of the prefilmer. The inner and outer air passages include air swirlers for imparting swirl to compressor discharge air flowing therethrough. As fuel exits the swirler/prefilmer, it is sheared between the swirling air flows issuing from the inner and outer air passages to atomize the fuel for combustion.
While pure airblast nozzles can provide for clean fuel combustion when the engine compressor is spooled up, difficulties can arise during engine startup. Pure airblast nozzles depend on fast moving air to break up the slower moving fuel spray into fine droplets. As described above, the airblast typically comes from compressor discharge air routed through the nozzle. However, during engine startup the compressor is not fully spooled up and thus the air pressures provided to the nozzle during engine startup are not always high enough to provide the necessary atomizing air blast. Therefore, the amount of fuel atomized can be insufficient to initiate or sustain ignition. Thus it can be quite difficult to start an engine using only traditional pure air blast nozzles.
Another problem during startup for traditional airblast nozzles is that when the startup air pressure is too low to fully atomize the fuel flowing from the nozzle, significant amounts of fuel can issue from the nozzle without being atomized. Liquid fuel drooling from the nozzle constitutes waste of fuel and can cause poor emissions as well as complications that can arise from fuel pooling in undesirable locations of the engine. Pooled fuel can ignite explosively and emit a plume of white smoke out of the exhaust.
Some solutions to these problems have been suggested, such as including auxiliary start nozzles, for example liquid-pressure atomizing nozzles, dedicated for use during start up. It is also known to use hybrid nozzles, which include air blast fuel nozzles for full power operation in addition to liquid-pressure atomizing nozzles for use during startup. Piloted airblast nozzles are sometimes used to achieve the needed starting characteristics while trying to match pure airblast nozzle performance. However, piloted airblast nozzles tend to lack the superior thermal management inherent in pure airblast nozzles. Piloted airblast nozzles also fail to achieve identical spray characteristics with pure airblast nozzles because the pressure atomizing circuit mixes with the airblast spray.
Other solutions include adding auxiliary air pumps or compressors to generate atomizing air blasts through pure airblast nozzles during engine start up. However, while these solutions can be used to facilitate engine start up, they can also add significantly to the cost and weight of the engine.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for staged pilot injectors that allow for improved staging pressure ratios for lean direct injection. There also remains a need in the art for improved airblast injectors with improved fuel distribution at low power levels, such as for improved startup. The present invention provides a solution for these problems.